Research Highlights

A Sweet Solution to Sour Gas

Deadly hydrogen sulfide must be removed from natural gas before it can be used. A new chemical solvent from PNNL promises to make the process more effective and efficient.

Along with affordable and reliable energy, natural gas can harbor an extremely toxic threat—hydrogen sulfide—which must be removed before the gas can be used safely. In a surprising twist, PNNL researchers in search of carbon dioxide capture solutions have discovered a way to remove hydrogen sulfide from natural gas using a water-free solvent that promises to be much more effective and efficient than current techniques.

"We hope that this is a new ‘sweetening' solvent, so to speak, where we're able to be more energy efficient and more selective at removing hydrogen sulfide," said Dave Heldebrant, who is the principal investigator for the project. "It's a way to improve the safety and purity of the gas, but at the same time be more energy efficient."

Why it matters

Natural gas contains varying levels of hydrogen sulfide, so-called "sour gas" for its poisonous properties, that can be deadly even at low concentrations and corrosive to pipelines and equipment. This research delivers a new hybrid chemical/physical solvent for removing hydrogen sulfide that is three times more selective than the industry standard solvent and is much more energy efficient—saving dollars, conserving energy, and potentially opening up natural gas resources worldwide.

Methods

Initially, researchers were investigating methods to remove CO2 from natural gas using a water-free chemical solvent--significant because chemical-based CO2 removal from natural gas currently requires millions of pounds of water-based amine solvents per hour that must be boiled to release the CO2. That requires a lot of energy. Alternative methods include forcing the CO2 to dissolve into a "physical" solvent using the high pressure of the natural gas as the driving force.

"Processing chemical solvents in water on an industrial scale is extremely energy intensive, and physical solvents generally have lower selectivity," Heldebrant said. "Our thought was: Can we make this process better by first getting rid of the water and somehow combining the two processes?"

Researchers began with a base and alcohol combination that traditionally does not react with CO2 at atmospheric pressure, but when they added higher pressures of CO2, the blend chemically reacted with the CO2 to make a liquid carrier. Researchers then reduced the pressure and observed the chemically reacted CO2 being released from the solvent without heat. This first-of-a-kind hybrid approach combining chemical selectivity with pressure-based CO2 removal delivered a major breakthrough: demonstrating that CO2 could indeed be removed from natural gas without water.

Encouraged by the results with CO2, researchers moved on to other naturally occurring compounds found in natural gas, including hydrogen sulfide. That's when they really hit the sweet spot, discovering that the water-free solvent was even more effective at hydrogen sulfide capture.

"Nobody had ever seen a chemical solvent react with hydrogen sulfide in the absence of water and still remain a liquid. It had never been published," Heldebrant said. "We set out and proved that the reaction was occurring and the produced liquid further absorbed more hydrogen sulfide, bringing the total capacity up to 43 percent by weight. You're getting the same capture that you would have in the water-based solution, but now you don't need the water."

Analysis under real-world natural gas processing conditions has confirmed the initial results and indicates that the hybrid solvent has excellent potential for hydrogen sulfide removal, absorbing it 10 times more selectively than CO2 and having a much higher selectivity for hydrogen sulfide than the industry standard solvent. What's more, this unique hybrid solvent favors hydrogen sulfide removal, while all other solvents favor CO2.

What's next

Subsequent research will focus on further refining the solvent chemistry for optimal performance, and demonstrating the technology on a bench scale.

Acknowledgements

Funding for this work came from PNNL. Wiltec Research Company performed the materials testing. The Fluor Corporation performed the data analysis.